1) Field of the Invention
The present invention relates to a write once optical recording medium using a phase-change in a recording layer.
2) Description of the Related Art
A phase-change type optical recording medium is well known. In such recording medium, information can be recorded and reproduced by changing a phase in a recording layer of the recording medium. The phase change is between a crystalline state and an amorphous state (non-crystalline state) which have a refractive index different from each other. In a conventional manufacturing process of the phase-change type optical recording medium, the recording layer is deposited on a substrate by means of a thin film process under a vacuum such as a vacuum deposition method, an ion-plating method and a sputtering method. The recording layer on the substrate after the deposition process has an amorphous state. Then, a beam of a laser, halogen lamp or the like is irradiated on the substrate so that an optical recording medium having the recording layer in a crystalline state is provided. (The aforementioned process to crystallize the recording layer from the amorphous state into the crystalline state is hereinafter called an “initialization step”.)
In order to record an information signal to such optical recording medium the recording layer in a crystalline state is irradiated with a laser beam so that the phase in the recording layer is changed from the crystalline state into the amorphous state. Thus, the signal can be recorded by a recording mark which is formed as above.
With respect to the manufacturing process of a write once optical recording medium, which is capable to record the information only once, it is preferable that the phase of the recording layer can be easily changed from the amorphous state into the crystalline state during formation of the crystalline state of the recording layer which is the state before recording the information signal. To the contrary, it is preferable that the phase of the recording layer cannot be easily changed again from the amorphous state into the crystalline state in order to prevent a degradation of the information signal after recording the information. Thus, it is preferable that the recording layer before recording the information signal has stability in the crystalline state whereas the recording layer after recording the information signal has stability in the amorphous state.
Japanese Patent Kokai No. 11-73692 discloses a method for manufacturing a write once optical recording medium. The recording layer in the optical recording medium is directly deposited in a crystalline state, not an amorphous state, on the substrate during the step of forming the recording layer on the substrate. Thus, the initialization step can be omitted. This recording layer has a multi-layer structure comprising a first recording layer consisting of a Te-series material and a second recording layer consisting of a (Ge, Sb)-series material. In recording the information signal, a power source for optical recording applies power to the recording layer, so as to melt and mix the first recording layer and the second recording layer. When the optical recording power is stopped, these layers are cooled down rapidly and solidify in the amorphous state. Specifically, the phase of the recording layer is changed from the crystalline state into the amorphous state by an on-off operation of the optical recording power. Thereby the information signal is recorded. In the manufacturing process for such a write once optical recording medium, it is necessary to purify Te in the materials since the first recording layer made of Te series material is deposited in the crystalline state on the substrate without undergoing the amorphous state. The first recording layer is particularly vulnerable to oxidization and is difficult to handle.
A method to record the information by forming a crystalline recording mark is also known. The recording layer in the amorphous state is irradiated by a laser light to change the state of the phase. Thereby the information signal is recorded. However this method is not preferable for the write once optical recording medium because of the difficulty in edge control between a signal recording area and a matrix which tends to cause increased jitter.